Energy absorbing restraints can be used in a many applications. For example, such restraints are used as restraining devices for process piping to dampen motion of such piping caused by dynamic events such as fluid transients, flow induced steady state vibrations, earthquakes and the like.
Additionally, for thermal considerations, piping systems must be supported to allow expansion due to the thermal growth induced by a hot process fluid flowing therethrough. Design conflicts occur however, where piping system restraints are necessary to limit dynamic displacements at locations along the pipe which also encounter thermal growth.
Traditionally this conflict has been resolved by using snubbers which allow the pipe to freely expand but momentarily restrain the pipe during a dynamic event. Snubbers, however, absorb little energy, must be periodically serviced and have been known to fail, resulting in costly snubber inspection programs, particularly in the nuclear power industry.
Various alternative restraints have been proposed, including gapped restraints and energy absorbers. Gapped restraints allow free thermal travel of the piping system, but limit dynamic travel to the limits of preset travel constraints. One disadvantage of gapped restraints is that they impart high impact loads to the adjoining structure when such a restraint reaches the ends of its travel during a dynamic event.
Several types of energy absorbing restraints are available. One example of an energy absorbing restraint is disclosed in U.S. Pat. No. 4,620,688, to Khlafallah et al., entitled "Energy Absorbing Apparatus For Piping System And The Like", which utilizes steel flex plates which act as a spring. The spring action allows for thermal expansion and also absorbs energy by plastic deformation of the plates during dynamic movement. Such a restraint, however, has a low cycle fatigue life which is a significant drawback.
Another type of energy absorbing restraint is disclosed in U.S. Pat. No. 4,955,467, to Kallenbach, entitled "Energy Damping Device" which discloses a device where energy is absorbed by friction which is generated in a reciprocating piston and cylinder arrangement. A significant drawback to this type of restraint is the large amount of variability in the resulting frictional forces, which provides inconsistent energy absorption.
Still another type of energy absorbing restraint incorporates multi-strand helical cables trapped between plates which function as a spring to allow for thermal expansion. Devices using such an arrangement are disclosed, for example, in U.S. Pat. No. 4,190,227, to Belfield et al., entitled "Vibration Isolator And Method For Manufacturing Same," and U.S. Pat. No. 4,783,038, to Gilbert et al., entitled "Isolator Apparatus."
These devices absorb energy by the rubbing and sliding of cable strands when such strands are subjected to dynamic displacements. The energy absorbing component of such devices is known as a wire rope isolator.
U.S. Pat. No. 5,240,232, to Loziuk, entitled "Pipe Restraint," which patent is commonly assigned herewith, discloses wire rope isolators incorporated into a pin-pin device for connection between, for example, a process pipe and a structure.
That device uses wire rope isolators which trap or clamp the wire cable bights (i.e., coils) at about 180.degree. degrees apart. Such devices, however, typically require centering bushings or slide plates to maintain the moving portions of the device coaxial, one relative with the other.
Accordingly, it is advantageous to have a device which uses wire rope isolators configured so as to eliminate the need for centering bushings or slide plates, which device maintains the moving portions of the device coaxial, one relative with the other.